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Using Classes

Using Classes. Lisp also supports the Object-Oriented paradigm. The process of defining classes in Lisp is similar to how you you define structures. The syntax of defining classes are as follows: (defclass Class_name ( Superclass* ) ( Slot_definition* ) Class_option* ).

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Using Classes

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  1. Using Classes • Lisp also supports the Object-Oriented paradigm. The process of defining classes in Lisp is similar to how you you define structures. • The syntax of defining classes are as follows: (defclass Class_name (Superclass*) (Slot_definition*) Class_option* )

  2. Slot definition • Slots are properties belonging to the class. • Slots are lists containing: • Slot name, optionally followed by • Slot option keyword and associated value • You can provide an initial/default value of a slot, or you can set the value of a slot in your program during run-time. • Lisp allows you the flexibility of specifying how you would want to access slot values.

  3. A simple class • Here is a simple class defined in Lisp. (defclass Bus (Vehicle) ( (Make) (Color) (Year) ) (:documentation “The bus class”) )

  4. Class Vehicle (defclass Vehicle () () ) • This is the superclass for the Bus class. You would have to define this superclass first before defining Bus. • Vehicle is an empty class. It specifies no slots.

  5. Creating objects • To create an instance of Bus, you use the make-instancefunction as follows: (setq bus1 (make-instance ‘Bus)) • Recall that Bus has 3 slots , namely make, color and year. Now let’s look at some keywords that are used to initialize, read and write the values of these slots.

  6. Initialize Slot Value • To provide an initial value for a slot, use :initform (defclass Bus (Vehicle) ( (Make :initform ‘Ford) (Color :initform ‘Blue) (Year :initform 1989) ) ) (setq bus1 (make-instance ‘Bus))

  7. Reading/Writing slot values (1) • To read slot values, you can use the keyword :reader, which specifies the name of the function that will read the slot, while :writer specifies the name of the function to write to the slots. Continuing with the previous example: (defclass Bus (Vehicle) ( (Make :reader make :writer Set-Make) (Color :reader color :writer Set-Color) (Year :reader year :writer Set-Year) ) ) (setq bus1 (make-instance ‘Bus))

  8. Reading/Writing slot values (2) • Now let’s say that, we want to write values to the slots Color, Year and Make, the code below will perform this: (Set-Color ‘yellow bus1) (Set-Make ‘Ford bus1) (Set-Year 1989 bus1) • To read the value of those slots: (Color bus1) (Make bus1) (Year bus1)

  9. Using :accessor (1) • For each slot, specifying one function for reading its value and another function to write its value will be inefficient if there are a lot of slots. • It is better to just specify ONE function that is able to perform both read and write for a slot. You can do this using the :accessor keyword. • Going back to our running example …

  10. Using :accessor (2) (defclass Bus () ( (Make :accessor make) (Color :accessor color) (Year :accessor year) ) ) (setq bus1 (make-instance ‘Bus)) • To set the slot Yearto 2000, do this: (setf (year bus1) 2000) • Notice that setf is used to assign the desired value to the slot.

  11. Using :accessor (3) • To read the value of the slot Year, do the following: (year bus1) • Try this for the remaining slots.

  12. :initarg (1) • This is a way to pass in any initialization values for the slots during make-instance. The value specified using :initarg will override the value specified using :initform (defclass Bus () ( (Make :accessor make) (Color :accessor color) (Year :accessor year :initarg :year :initform 1989) ) )

  13. :initarg (2) • To pass in an initial value of year while creating an instance of Cars, do this: (setq bus1 (make-instance ‘Bus :Year 2000))

  14. Shared slot for instances of a class (1) • You can use the keyword :allocationto specify whether a slot’s value is specific to each instance of the class, or the slot’s value is shared among all instances of the class. • Now let’s create a subclass for our Bus as follows: (defclass SchoolBus (Bus) ((MaxSpeed :accessor MaxSpeed :initform 320 :allocation :class)) )

  15. Shared slot for instances of a class (2) (setq schoolbus1 (make-instance ‘SchoolBus)) (setq schoolbus2 (make-instance ‘SchoolBus)) (MaxSpeed schoolbus1) (MaxSpeed schoolbus2) (setf (MaxSpeed schoolbus1) 1000) (MaxSpeed schoolbus1) (MaxSpeed schoolbus2)

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